1. Field of the Invention
This invention relates to drill pipes for rotary-vibratory drills and, in particular, to joints for the drill pipes.
2. Description of Related Art
Rotary-vibratory drills employ a vibratory force superimposed upon a rotary action to accomplish the drilling operation. Sonic drills are rotary-vibratory drills where the vibration is in the sonic range.
Such sonic drills are used for such applications as drilling through overburden in placer exploration, installing concrete piles, water well drilling, rock drilling for blast holes and for rock coring.
One of the major reasons why sonic drilling machines have not been successful in the marketplace is the failure to develop suitable drill tooling. Extremely high alternating forces are generated within the drill pipe. Standard drill pipe is designed to withstand the torque developed during rotary drilling, but not the high alternating tensile and compressive loads encountered in sonic drilling.
These reversing loads are especially critical at the threaded ends of tool joints because of the stress concentration created by the presence of the threads.
Various approaches to solving the problem of joint failure have been attempted. One was to machine threads directly onto the drill pipe. This didn't work because the threads reduced the cross-sectional area of the pipe and simultaneously acted as a point of stress concentration as suggested above.
Another attempt was to use heavy wall pipe. However, it was then realized that the drill pipe stains or elongates and contracts the same amount in resonance regardless of its thickness. The greater cross section simply increases the force which must be transmitted by the joint, leading to failure at the 40 threads.
Next, drill pipe was tested with a relatively thick tool joint in relation to the thickness of the drill pipe. This configuration is routinely used for rotary drills used in drilling oil and water wells. These are generally assembled by circumferential welds between the pipe and threaded connection members used at the joint or by friction welding the members to the pipe. These welds however do not long withstand the fatigue loading conditions and high stress concentrations encountered in sonic drilling.
Pipe joints employing alternating fingers and slots extending circumferentially about the pipe have been used for other purposes such as the oil drilling platform disclosed in U.S. Pat. No. 3,521,811 to Bardgette. However, the problem encountered in that instance is not analogous to the difficulties encountered in sonic drilling and therefore does not suggest a solution to the problem.